Minecraft Server Won't Start: EULA, Java, Port and Memory Fixes

Read the console first

Open your server on panel.chicagoservers.co and look at the Console tab right after a failed start. Scroll to the first error and copy it. Many later lines are consequences of the first. The table below matches what you see to the section that fixes it.

EULA not accepted

The server will not run until you accept Mojang's EULA. The message is "You need to agree to the EULA in order to run the server. Go to eula.txt for more info."

1. Open eula.txt

   Go to the Files tab and click eula.txt in the server's main folder.

2. Accept it

   Read the EULA, then change the value so it says:

   Copy

   eula=true

3. Save and start

   Click Save, then start the server again.

Wrong Java version

UnsupportedClassVersionError means the server was built for a newer Java than the one running it. The message names two class-file versions, and the major version is the Java version plus 44. For example, class file 69 is Java 25.

Choose Java on the Mods tab, which also sets the server software. Do not downgrade Minecraft to fit an old Java. Mod loaders and plugins need to match as well, so check installing Forge and Fabric mods if you are modded. Older Minecraft versions need older Java, so follow the version's own requirement.

Port already in use

The console shows the server starting on an address and port, then "FAILED TO BIND TO PORT!" or "Address already in use". The usual causes are an old process that has not fully stopped, or a hand-edited server-ip or server-port in server.properties.

• Stop the server completely, wait a moment and start it again.
• On ChicagoServers the panel assigns the address and port, so leave server-ip blank and do not change server-port.
• If it keeps happening, contact support with the console output.

Out of memory

• OutOfMemoryError or "Java heap space": the server needs more RAM than it has. Check how much your players, mods and view distance need in how much RAM a Minecraft server needs.
• The log just stops, sometimes in the middle of a line, with no error and no crash report. That can mean the server was stopped for using more memory than it is allowed. Treat it as a memory problem.
• Lower simulation-distance and view-distance, and remove mods or plugins you do not need. See the lag guide.

Damaged world or a world from a newer version

• "Failed to load world data. World files may be corrupted." means level.dat cannot be read. Restore a backup from the Backups tab. See the backup guide.
• "This world was created by an incompatible version" means the world was saved by a newer Minecraft. Worlds cannot be downgraded, so run the version that saved it or restore a backup from before the upgrade.
• Some hosts suggest replacing level.dat with level.dat_old. Only try it on a copy, with a backup in hand.

Bad plugin, mod or datapack

• Plugin or mod: a failure while loading is reported as a stack trace. Find the first package that is not net.minecraft, since that names the culprit. If none is named, remove the most recently added file and try again, or disable them in halves to find it. NoClassDefFoundError, ClassCastException and NoSuchMethodError usually mean a plugin or mod that does not match your Minecraft or server version.
• Datapack: the message "Failed to load datapacks, can't proceed with server load" says you can fix the datapacks or reset to vanilla with --safeMode. That is a launch option, so contact support and we will start it for you, then remove the faulty pack.

Other start problems

When you contact support, send the first error from the Console tab, the Minecraft version and server software, and what you changed last. Looking for a host? See Minecraft server hosting.

Frequently asked questions